Pulse multiplex communication system



Nov. 30, 1948. D. D. GRn-:G

PULSE MULTIPLEX COMMUNICATION SYSTEMk Filed Aug. 19. 1944 4 Sheets-Sheet l TF6/VSL@ TOR PULS E SOURCE I N V EN TR. w/VH D. [iF/EG BY /K Nov. l30, 1948. D. D. GRIEG 2,454,792

I PULSE MULTIPLEX COMMUNICATION SYSTEM Filed Aug. 19, 1944 4 sheets-sheet 2 lINVJENTOR. HOIVHZO D. GIP/E6 v BY c7 Arran/EY 4 Sheets-Sheet 5 nou/La- -rae D. D. GRIEG PULSE MULTIPLEX COMMUNICATION SYSTEM CIT/PP//Vf LEVEL GENEIPHWR .sain/HL v aou/vc: cHn/v/vsc /ve.l F

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Filed Aug. 19, 1944 ATTRNEY D. D. GRIEG IULSE MULTIPLEX COMMUNICATION SYSTEM Nw. 3o, 194s.

4 Sheets-Sheet 4 Filed Aug. 19, 1944 INVENTOR 00A/mo o, Gels@ ffy/?? voltages.

Patented Nov. 30, 1948 PULSE MULTIPLEX COMMUNICATION SYSTEM Donald D. Grieg, Forest Hills, N. Y., assigner to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application August 19, 1944, Serial No. 550,184

17 Claims.

l This invention relates to communication systems and more particularly rto multi-channel signal communication systems and methods employing a plurality of pulse trains for transmitting 'and selectively receiving various intelligence.

In intelligence conveying systems and more particularly in .pulse communication systems, intelligence may' be'transmitted by trains of short impulses modulated in `accordance with signal According to these systems the modulation may be by displacement in time or with respect to other pulse characteristics.

, In the time displacement modulation oi pulses, 'as in one instance disclosed in the copending application of E. M. Deloraine and J. L. Fearing, Serial Number 506,802,1iled October 19, 1943, now Patent No. 2,429,613, dated October 28, 1947, entitled Transmission systems, describing a multichannel signalling system, 'the pulses for each channel may be paired ofi, the time interval between these pulses beingA smaller than the time @interval between succeeding pairs of pulses. The different trains of channel pulses are differently timed so that when they are fed to a common transmission link, they are interleaved together with a given time spacing between succeeding pulses. At regular intervals the train of pulses is interrupted to form groups of pulses usable for synchronization at the receiving end. At the receiving terminal means are provided to segregate the respective channels from the incoming transmitted pulse trains. The pulses of a segregated channel are then applied to a relatively simple demodulator for reproduction of the intelligence conveyed by such channel.

In other instances, the pulses of the various channels are interleaved by means lof quite regular sequential timing, necessitating more involved 4demodulating means at the receiving terminal as compared tothe simple demodulator employed for irregularly timed pulse trains.

In order to render the facilities of multiplex systems of this type more fully available, it is frequently desirable to be able to shift, translate the spacingor offset the different channel pulses with krespect to one another and along the time axis, either to ll in unused spacesbetween groups `of pulses or pulse pairs, or for the purpose of varying the relative timing position of the channels.

'The latter may be ofradvantage, for instance, when trying to .preserve the privacy of communication, i. e. to avoid undesired detection of any l given one or a plurality of channels, or where rearrangement of the spacing of the channels from a yregular to an irregular one is desirable,

' Well as at the receiving terminal.

An irregular spacing is advantageous in that it makes it possible for a relatively simplified demcdulator to be used at the receiving end, as Will appear hereinafter.

The translation from regular to irregular spacing, of course, may be also had at the receiving terminal, as in many systems it is preferable to transmit uniformly spaced pulses.

In accordance with my invention, one or more circuits, for translating the position of pulses which may be of the type disclosed in my copending application Serial Number 522,139, entitled Translator system, iiled February l2, 1944, may be provided, as required, at the transmitter, as

A pulse position translating circuit may be provided for each or any of the channels to be used, as desired, at the transmission end, While at the receiving terminal, I provide pulse position translating circuits for varying the relative time position o the channel pulses themselves, or as a means for obtaining a channel segregating deblocking signal from a locally produced pulse train.

the invention will become more apparent upon consideration of the particular description thereof made with reference to the accompanying drawings, in which:

Fig. l is a block diagram of the transmitting terminal of a multi-channel communicating system incorporating pulse position translating circuits in accordance with the invention;

Fig. 2 is a cusper type modulator circuit for one of the channels of Fig. 1;

Fig. 3 is a typical pulse position translator vcircuit shown in block form;

Fig. 4 is a series of graphs illustrating the operation of the cusper modulator circuit of Fig. 2, the tvpe of pulse train obtainable from the circuit of Fig. l, and the operation of the translator circuit of Fig. 3;

Fig. 5 is a block diagram of another form of transmitting terminal in accordance with the invention;

,'Fig. 6 shows a number of graphs indicating mis- Vnelsillustrated in the figure.

cellaneous pulse train relationships as obtained from the circuit of Fig. 2 and correlated to the receiving circuits of Figs. 7 and 9;

Fig. '.7 is a block diagram of the receiving terminal of a multi-channel signalling system having means for translating the position of the pulses of each` of the channels;

Fig. 8 is' a schematic diagram of a demodulator circuit for translating the time modulated pulses into amplitude modulations;

Fig. 9 is a modification of the receiving terminal of Fig. 7.

Turning first to Fig. 1, the reference character I' designates a control generator for producing a basewave which may have anydes'ired Vfre quency. Energy from the source IA is applied directly to a modulator 2 and over phase shifters 3, 4 and 5 to time modulators 6, 'l and, respectively, representing the circuits of the four chan- Separate signal sources 9, I0, II and-I2 apply modulating signals to the modulators 2,6, 4'I and 8 to provide, for

instance, time modulated pulse trains for a transmitter I3 in accordance with thesignals. A pulse 'position vtranslator i4 is provided in channel 4 to shift themodulatedfpulses of the modulator 48, as desired.

The modulators 2,56, i and 8 may be of any one 'of several forms wherebya basic train of pulses,

as obtained :from the wave vproduced at thegem erator l, may be time modulated according to the 'signal fromthe signal sources 9,110, II yand I2. fHo'wever, to illustrate the utility of a pulse position translator inl connection with ,azspecic exampla'a modulator `will be described hereinafter, Vasdis'closed in the copending application Serial `I\To'1inlo`e"r506,802 by E. M. Deloraine yand'J. L. Fear..

ing,file'd October 19, `1943, and as shown in detail'in Fig.`2. The'modulator of Fig.2, 'aslwill appear presently, is-a so-o'alled biased push-pull ptyp'e of modulator. There are available a number of other suitable pushepull types of modulators,

for example the cusper types disclosed in the copending `joint application y'of E'. Labin and D.

'DJ Grieg, Serial 'Number 455,897, led August 24,

`1942, now Patent No. 2,416,329, dated Feb. 25, 1947,'or the gate Aclipping t'ype disclosed in a second joint application of E.'Labin and D; D. Grieg, Serial Number 455,899, also 'iiled August 24, 1942,

now Patent No. 2,387,969, granted October 30, "1945.

In'the modulator circuit-'of Fig. 2, the modulating signal wave from the` signal source 9 is ap- 'plied to `an'arnplitude limiter I5. The output of the amplitude limiter I5zfeeds the primary of a transformer' it between the two windings II-and I8 of which is connected a direct current, biasing source of potential I9 having inshunt therewith a potentiometer 20. In series with each of the two-'secondary windings and I8 is connected one of the two secondary windingsZZ-and -23 of an input transformer 24 adaptedto receive the base" Wave 'energy IW, graph a Fig. 4, from generator I, either directly as in channel I or through a phase shifter as-in channels2, 3, and VI such as shown for threelchannels in the graph a of The'operation ofthe cusper of Fig. 2, as'disclosed in the application Serial Numberi506,802 referred to hereinabove, k.will befbriefly recapitulated in the present instance. Any of the time control waves IW, 2W or 3W shown in graph a, differing in phase from each other by the action of the phase shifters in each of the channels, is applied to the transformer 24. The potentiometer 20 controls the Ibiasing of the secondaries which may beat the level 32' (graph a). The full wave rectication of= the waves yas obtained at the load resistor 3|', thus occurs with reference to said level 32, as the axis of rectication, thereby producing cusper output Waves 33, 34 and 35 having eusps'36, 31 and38, etc., graph b. These cusps are paired oir in time, the interval t1 between the cusps of each pair'being smaller than the interval itfbetweensucceeding pairs of cusps of the same channel. The -relationship of the two intervals represented byfti and t2 is determined in part by the selected bias on the cusper as well as by the number of channels to be contained in one period of the time controlled-wave, such as represented `v`byLIW. The.v shape of the pulses delivered by the l modulator-is determined -by a pulse Shaper cir- -cuitwBS-into which'the `output of the cusper circuitf'21risrfed. The adjustment of a control grid resistor'A and a` cathode-resistor determines 1thewidth ofthe-resulting output pulses as well as their relative flatness. By inspection of graphs Pb-,andc (Fig-4), the-pulses corresponding to the variouscusps :will berecognized, as for instance pulse Ia Whichisthel resultant ciY the cusp 36. In

'viewrof the'small and Alarge-wave portions on opposite sidesottheaxis 32, `the pulses of the three fchannelsformggroupsgotsix,pulses leaving an interval'f42therebetween As disclosed in applireceiving-station. In-this instance, however, the

- insertion or sandwiching in of anadditional channel for-signalling or monitoring purposes is proposed, .as illustrated in connection with channel lv.4 of Figui. -For-thisreason the pulse position translator circuit: I4 in channel 4 (Fig. 1) has been prcwided.`

IThe pulse position translatory ycircuit may be ofthe type disclosed in the copending application SerialNumber 522,139 mentioned hereinabove, vandfasv shown in block form in Fig. 3. The pulse .position translator receives a series of` pulses from .asouroesuch as the modulator 8 vin channel 4 of-Fig. l. This pulse train is comprised of pulses which are paired 01T asingraph c, and are referred to in graph d of Figui. as 4a and 4b, showningtheir relationship with `respect to the pulse trains of channelsl, 2and 3,t0 indicating the shiftin phase dueitothe phaseshifteri. Pulse lfb isfirst'positioned in; time interval 42 as nally required, and pulse-.4a.is then shifted toward 4b by means ofithe` translator circuit I4 by a time c-intervalftpto aposition 4aI, -Which places pulses 4a and 4b into the interval 42 graph c,.Fig. 4, as

desired. lThe:operativefeifect of the translator circuit-dependsfon adelay t4 giventhe leading pulse Allafbyfmeans:of a multivibrator circuit 44 (Fig. l3) `whichisenergized by the leading pulse 4a to yproduce a vfrequency dividing pulse 45 v(graphe,Fig-4).

The Width 'of pulses 1 45 may be adjustedby suitablyy varying the proper parameters of-` the circuitf 44 and thereby controlling the amount of. delay effected thereby. AFrom the mulf'bina-tion of-pulses 4a and 4b and of thefpulses 45 produces awaveforinsuch'as shown in graph f '(Figwi'). vThis Waveform isthen clipped at levels 'demodulating tube 19.

41 and 48 in a clipper circuit 49 and diierentiated in a circuit 50, the result of which is shown in graph g (Fig. 4). These pulses are then clipped again and amplified in a circuit 5l at a level 52,

producing a series of pulses Aal and 4b, in accordance with graph hof Fig. 4 which are now offset in such a manner as to fall within the interval 42 (graph c, Fig. 4). Any desired translation in the position of the pulses may thus be achieved in the manner just described.

Another possibility for translating the position of pulses at thev transmitter end of a signalling system is illustrated in Fig. 5, wherein each of the four signalling channels is provided with a pulse position translator I4, otherwise being generally similar to the circuit of Fig. 1. However, inthis instance, the individual modulators 2, 6, land 8 are not biased as in the case of Fig. l

' and in fact are totally unbiased except for the periods when modulating voltages are' applied. d

This will result in regularly spaced pulses being supplied from the modulators of the type indicated in graph a., Fig. 6. By the proper adjustment of the phase Shifters and modulators in each of the channels of Fig. 5, another grouping of the trainsof pulses as shown in graph b, Fig. 6, may be obtained, if desired. Another possible result obtainable by manipulation of the circuit of Fig.l 5 is shown in the pulse train of graph c, Fig. 6. Other variationsrare possible as required.

Fig. '1 illustrates in block form a receiving circuit for receiving'regularly spaced incoming pulses, such as shownrin graph e, Fig. 6. Here, the pulse trains, coming in regular sequences, are received in a receiver 53, and are from there fed to a base pulse generator 54, wherethey are effective in exciting a basic pulse undulation havinga repetition frequency directly proportional to that of rthe incoming regularly spaced pulses of one of the channels'. The base pulse wave, thus obtained, is applied in 'turn to each of delay circuits 55, 5B, 51 and v5&3 where the base pulse is given a suitably' timed delay in phase, to be used as a channel deblocking and segregating signal in the mixer circuits'f 59, 65, 6l Yand 62, to which the incoming pulse trains from the receiver 53 have been applied simultaneously.Y The resultant of the'mixer circuit 59 is shown in graph f of Fig. 6, corresponding to the pulse trains of graph a (Fig. 6). These output pulses are then clipped in clipper circuits 63 through 6B, the respective channels thus becoming separated. In order to v make it possible, as brought out hereinabove, for

simplined time displacement demodulators 61 through 10 to be used, pulse position translators 1I through 14 are provided in each of the channels'preceding the demodulators.

v Asimple form of such a demodulator is shown in Fig. 8. The pulses are applied over coupling kelements and 1E to an input grid 11 of a These pulses serve to shock excite a circuit 19 tuned to a harmonic of the cadence frequency of the input pulses. This ywave generated in circuit 19 is combined with the pulses applied at the grid 11 in controlling the output of the anode electrode 89, raising in effect the-pulses to different amplitude levels in accordance with the time displacement modulation. The demodulator shown by way oi example is not a part of this application and so its operation is not explained in detail. For a complete understanding of the operation of this demodulator circuit, reference is had to my copending appli- "cation 459,959 of September 28, 1943, now Patent No. 2,416,306, granted February 25, 1947, entitled Demodulaton These demodulated pulses are then passed on to an audio reproducer or other suitable type of amplitude modulation responsive equipment. Without going into the details of the operation of the demodulator of Fig. 8, it will suice at this time to point out that the circuit, which is quite simple, will operate bestwhen the time modulated pulses applied to its control grid are irregularly spaced or oiset rather than being spaced regularly as shown in graph f, Fig. 6. By proper adjustment of the corresponding pulse position translator circuit, every other pulse of the pulse train shown may be displaced or offset to form a train of paired-ofi pulses as illustrated in graph g, Fig. 6, the amount of time displacement being indicated by td. The displacement is attained in a manner as has been described in connection with Figs. 3 and 4.

A synchronizing feed back connection 8|, Fig. 7, is provided from the output of the demodulator 61 to the base pulse Wave generator 54 serving to keep the generator from drifting out of synchronism with the cadence frequence of the incoming pulses.

Should the incoming pulses be of the displaced type shown for instance in graphs b and c of Fig. 6, it becomes necessary to translate in position or offset the deblocking signal pulse in order to segregate the individual channels. The circuit ior doing this is shown in block form in Fig. 9. The incoming pulse trains applied to a receiver S2 are effective in exciting a base pulse wave generator 83, as explained in connection with Fig. 7, to produce a base pulse wave. The

base pulse Wave generator may be, for instance, the type known as a blocking oscillator, the principles of which are well-known and will not be described in further detail. The base pulse wave, thus obtained, is applied over the respective pulse position translators 84 to 81, where in each case it obtains a suitable translation in the base pulses corresponding to the offset of the pulses of the respective channel. To the translated base pulse of each channel is then given, if necessary, a proper shift in phase or timing by means of the delay devices 88 to 9i to effect a complete synchronization of the basic pulses with the desired channel pulses. The basic pulses, now positioned to properly segregate the respective channels, are then applied to mixers 92 through 95, there serving to deblock for instance channel I, in the manner shown in graph d, Fig. 6, from the pulse trains or" graph c, Fig. 6, the deblocked pulses then being clipped at a level just above the deblocking pulses, as in graph d. The pulses for the various channels obtained in this manner may still be regularly spaced as far as the ultimate demodulator is concerned, and the receiver of Fig. 9 is therefore provided with additional translating devices 96 through 99 in the four channels in order to be able to translate the individual channel pulses obtained by deblocking into a desired offset relation for demodulation. rlhe demodulators |69 through |03 may be similar to those discussed in connection with the receiver shown in Fig. '1. A synchronizing feed back connection 04 from the output side o-f the demodulator S00 to the base pulse wave generator 83 is provided, similarly to that shown in Fig. 7.

It is apparent from the above description that I'have provided a communicating system of extreme exibility wherein the relative oiset of the pulses of the various channels is quite independent at either terminal of the lsystem from that of the other terminal.

It is to be distinctly understood that the ernbodiments of my invention specifically described herein are givenby way -of example only and are not to be regarded as limitations on the scope of my invention, as set forth in the objects thereof and in the accompanying claims.

I' claim:

l. A 'multi-'channel Vcommunicating system comprising means for producing for each of a plurality of channels a separate pulse train time modulated in .accordance with a corresponding signal, means to time differently the pulses of the different lchannels to interleave the different pulse trains together as a single train of pulses,

`pulse position translating means in at least vone of said channels, means for transmitting said interleaved train of pulses over a common transmitting medium; pulse receiving means having a plurality yof receiving channels; a plurality of channel pulse train segregating means, one for each ofsaid receiving channels; pulse position translating means in at least one of said receiving channels, and demodulating means in eachl of said channels.

2. 'Apulse communicating system comprising means 'for producing a train of pulses modulated in accordance with a signal, means to translate the position of said pulses, means to transmit said pulses, means to receive `said pulses, means to translate the position of said received pulses associated with said receivingfmeans, and means I or demodulating said pulses.

3. A multiplex signalling system comprising means for .producing a train f pulses for each Iof a .pluralityfof channels, means biasing the operation offsaid pulse producing means to pair olf the pulses, the pulses of each pair, when unmodulated, having a, time interval therebetween different from the time interval between the succeeding pairs 'of pulses 'of the same channel, means to time diierently the pulses of diierent channels means for applying the trains Iof pulses toa transmission medium common to all channels, whereby the pulses are interleaved together and, because of the paired-off relation of the pulses, form -distinct'groups of pulses with a given time vinterval between the groups; means for producing at least one pulse channel additional to the above named channels, means to translate vthe :position of the :pulses of said additional channel, whereby the pulses thereof may be sand- Wiched into the time intervals between said groups of pulses.

4. A multiplex signalling syst-em comprising means 'for producing a train of pulses for each of a plurality of channels the pulse Itrains and channels being related in time in accordance with a base wave and modulated in accordance with a corresponding signal, means for transmission of said pulses, common to all channels, means for receiving 'the output -of said common means, mea-nsresponsive to saidoutput for deriving a control wave corresponding to saidibase wave -channel-segreg-ating means responsive to said control wave forming `a plurality of receiving channels corresponding to the transmitted channels, means in each of said receiving channels to translate the relative spacing of the pulses of such channel, means to `demodulate said translated pulses `in eachchannel, fmeansior feeding back demodulation energy to the means vderiving said control wave.

A5. A multiplex'signa-lling system i-n accordance `with cla-lm l4, wherein said channel -segregating means includes base pulse energy generating means, vtiming means for said 'base pulse energy, and pulse mixing and clippingmeans in each of said channels.

6. Amultiplex signalling system in accordance with, claim 4, wherein said channel segregating means includes base pulse energy .generating means, and channel pulse train deblocking means comprising base pulse position translating means, timing means for said ibase pulse energy, and pulse mixing and clipping means-in the circuit of each of said receiving channels.

7. A multi-channel receiving system comprising means for receiving a plurality of series of pulses interleaved lto form a single pulse train, means to produce base energy pulses controlled by said receiving means; means for segregating the individual channels'from said pulse train including base pulse position translating means, means for delayingin phase said base pulses, means for mixing said received pulse train and said base pulses -delayed in phase, and clipping means for acting on the output of said mixing means, whereby the segregation of a series of pulses of a given channel is obtained; means for translating the relative position of the segregated pulses of said given channel; and means to demodulate the pulses of saidv given channel.

8. A method of multiplex communication over a plurality of channels, comprising producing a train of pulses for each channel displaced in time with respect to one another, modulating each of sai-d trains of pulses in correspondence with a. signal, translating in position the pulses of at least one channel, interleaving together said channel pulse trains for transmission over a common transmitting medium, receiving said interleaved pulse trains over a common receiving medium, and segregating said channel pulse trains for demodulation.

9. A method of multiplex communication over a plurality of channels, comprising producing a train of pulses for each channel the pulses and channels being related in time in accordance with 'a base wave, each channel modulated in accordance with corresponding signals and displaced in time one from the other, transmitting said trains of pulses over a common transmitting medium, receiving said trains of pulses, deriving from said received pulses a control wave corresponding to said base wave segregating said pulse trains in accordance with said control wave into individual channel trains, translating into different relative position the pulses of said individual channel pulse trains, demodulating said olset pulse 'trains and controlling the segregating means 'oy demodulation energy.

10. A method of Amultiplex communication over a plurality of channels, comprising producing a train of pulses for each channel, the pulses of each channel being paired-off, with the pulses of eachpairthaving 4a time interval therebetween diiTerent from the time interval between succeeding pairs of pulses of the same channel; modulating the pulses of each train in accordance with a signal; timing the trains of pulses, whereby the pulses of all the trains except at least one are interleaved together for transmission purposes and, because of the paired-oil relation of the pulses, form distinct groups of pulses with a given time interval between the groups; translating into oiset time position the pulses of said one of said trains to fit into the intervals .between said groups of pulses; transmitting said interleaved trains of pulses over said common medium; and at a receiving point, vreceiving said .interleaved trains of pulses segregating in accordance with individual channels said interleaved trains of pulses; and

E demodulating each of said segregated trains of plying the trains of pulses to said common medium for transmission with the timing of transmission different for each train, whereby the pulses of the different trains are interleaved together in said common medium, translating into offset position as. desired the pulses of the individual channel trains with respect to the other channel trains, whereby a desired relative offset relation among the pulses of the channel trains vis achieved; and at a receiving point, receiving said interleaved pulse trains over a common medium, segregating said pulse trains in accordance'with individual channels, and demodulating the segregated pulse trains of said individual channels.

.interleaved pulse trains over a common medium,

utilizing said received interleaved pulse trains for producing basic energy pulses, applying said basic pulses to each of a plurality of channels with the timing thereof different for each channel, using said differently timed basic pulses in each vchannel to segregate by deblocking and clipping the corresponding channel pulse train from the received pulse trains, translating in position the Y deblocked pulses in each channel to effect a pairing-off of the pulses in each train, and demodulating the paired-off pulses of each channel.

13. In a method of multiplex signalling of a plurality of channels in accordance with claim 12,

the step of synchronizing said basic energy pulses rby means of the train of demodulated pulses of y one of said channels.

' 14. A method of multiplex communication over a plurality of channels, comprising producing a. train of pulses for each channel, modulating the train for each channel in accordance with a signal, indiscriminately interleaving together said pulse trains of said channels, applying said interleaved trains of pulses to a common medium for transmission with the timing of transmission different for each train; receiving said interleaved pulse trains over a common medium, utilizing said received pulse trains for generating basic energy pulses, dividing said basic pulses into a plurality of series of pulses corresponding tov said plurality of channels, translating in position as required each series of basic pulses and varying the relative phasing thereof for alignment in time with a train of channel pulses, deblocking with said plurality of basic pulse series the respective corresponding channel pulse trains; clipping each delocked pulse train to obtain individual channel pulse trains, translating into offset position said segregated pulses; and demodulating the offset pulses.

15. In a method of multiplex signalling of a plurality of channels in accordance with claim 14, the step of synchronizing said basic energy pulses by means of the train of demodulated pulses of one of said channels.

16. A multichannel communicating system comprising means including a base Wave generator for producing and interleaving in time displacement series of pulses each series modulated by the signal of its respective channel, means for interpolating in intervals between pulses of said series, the series of pulses modulated. by the signal of an additional channel including devices under control of said base Wave for determining the interval and further means responsive to a pulse of the series to be interpolated to cause translation of pulses of the last mentioned series to conform to the interval.

17. A multichannel system according to claim 16 in which the said intervals are those occurring between sequences of pulses one from each channel.

DONALD D. GRIEG.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,089,639 Bedford Aug. 10, .1937 2,172,354 Blumlein Sept. 12, 1939 2,406,019 Labin Aug. 20, 1946 2,406,165 Schroeder Aug. 20, 1946 

